The drama of a young mother's act of betrayal during the last days of the Irish Troubles comes together with measured intelligence and artfully apportioned suspense in "Shadow Dancer."

The drama of a young mother’s act of betrayal during the last days of the Irish Troubles comes together with measured intelligence and artfully apportioned suspense in “Shadow Dancer.” British director James Marsh’s highly disciplined filmmaking costs this slow-burning IRA thriller a bit of narrative drive, and its taut but methodical accretion of details and revelations will play best to attentive viewers. But there is much here to savor, starting with a fine performance by Andrea Riseborough, which should stand the classy production in good stead in prestige festival slots and arthouse berths at home and offshore.

Utilizing a mobile camera that often follows the characters from behind or from side angles as they go silently about their business, Marsh immediately establishes a mood of quiet, watchful unease. A child growing up against the violent upheaval of 1970s Belfast, Northern Ireland, young Collette McVeigh (Maria Laird) sends her brother Sean (Ben Smyth) out to buy cigarettes for their father, a request she will regret for a lifetime when the boy is fatally shot during an exchange of fire between British and Irish forces.

The traumatic experience has the effect of radicalizing Collette, as becomes clear after a quick fast-forward to 1993. She’s now played by Riseborough, whose coolly grave expression brings a disquieting chill to a tense, near-wordless sequence in which Collette plants a bomb in a London subway station. But the authorities are on to her; the situation is quickly defused and Collette arrested, at which point MI5 officer Mac (Clive Owen) offers her the chance to become a mole for British intelligence.

Initially stubborn and defiant, Collette consents when faced with the prospect of being separated from her young son. She and Mac arrange to meet once a week along the Belfast coast, where she will supply details about the activities of her other two brothers, Gerry (Aidan Gillen) and Connor (Domhnall Gleeson), both high-ranking officers in the IRA. Not long after Collette’s return, Connor enlists her to participate in the assassination of a top Northern Irish police detective, providing her with the first test of her loyalties.

With patience and precision, the screenplay by Tom Bradby (adapting his own 2001 novel) builds a claustrophobic, closely observed portrait of Collette’s family life, in which paramilitary operations are inseparable from blood ties, and affection can all too easily curdle into suspicion. Living in the same house with her brothers and their mother (Brid Brennan), who observes their comings and goings with mildly reproving concern, Collette is all too aware of the danger she’s placing herself and her family in. But to do nothing would bring its own perils, and with an end to the Troubles seemingly imminent, her decision to exit this violent world with her son makes a great deal of sense.

Complicating the situation still further, Mac is determined to protect his young turncoat and her child, but encounters resistance from an icy superior (Gillian Anderson) who clearly has her own highly classified priorities. Bradby’s screenplay is implicitly apolitical as it shrewdly appraises the levels of secrecy and distrust on either side of the conflict, where any hint of insubordination or insurgency is met with swift investigation and punishment. Morphing into a detective story of sorts as it plays out the fates of the McVeigh clan, the film is also subtly perceptive about how easily a woman can be trusted, and therefore underestimated, in an aggressively male-dominated milieu.

Though best known for his documentaries “Man on Wire” and “Project Nim,” Marsh has tended to alternate between fiction and nonfiction, his most recent narrative entry being the second and strongest installment of 2009’s “Red Riding” trilogy. As in that film, the helmer works in a cleanly focused style to illuminate a world of matter-of-fact criminality in strikingly intimate terms. Inasmuch as it requires viewers to sift through confusing identities and layers of information, and also to lean forward to catch the often thickly accented dialogue, “Shadow Dancer” is admittedly slow to gather force and momentum over its 101-minute running time, though by the third act, the deliberately paced drama has exerted a hypnotic grip.

The occasional flareup of violence notwithstanding, it’s Riseborough’s performance that provides the primary source of suspense. Quick to act but slow to reveal her innermost thoughts, often clad in bright red to stand out against the surrounding countryside, her Collette makes a dramatic virtue of unreadability; the thesp is ably supported by Gleeson, Gillen and Brennan as the family members whose fortunes are so intricately entwined with Collette’s. Owen, the cast’s biggest name, is paired effectively with Riseborough despite a somewhat too hangdog expression at times.